I need to install 9, 6" can lights into a plaster ceiling. Attic acess is very good so I could drill a marking hole from below and actually cut from above to cut down on dust. Any sugestions would be welcome. I do lots of woodwork ,never encountered plater before. I say that to let you know that I have about any tool that you might suggest, so any sugestions are welcome.

Sawsall best tool for cutting plaster.

I'd be nervous about using a sawzall. It will have a bad finished edge and will be difficult to cut in a circle.

I'd use a hole cutting bit and do it from the ceiling going upwards. The bits are expensive but worth it. Start slowly from the ceiling side to get a clean edge. Something like this:
RIDGID No. 7033, 6 In. Bi-Metal Hole Saw - 7033 at The Home Depot you'll also need the center bit that this mounts on.

It's a lot easier if you can layout your lights so you do not hit a joist when you drill the hole.

I had seen that hole bit and thought that might be the way to go. You don't think it will burn out before 9 holes are cut, even with occasional water dunkings?

It should be OK for 9 holes - you're only going through plaster and lath.

It's easiest to layout pot lights with string lines along the ceiling to get everything square and straight. If you are concerned about patching nail marks then I suppose you could use tape or assistants to hold the string in place... or you chalk lines if you don't mind wiping up.

The trick is going to be to ensure that you can lay it out & then drill without going into ceiling joists. Plaster and lath will make the joists more difficult to find with a stud finder. If you can't find them accurately, then perhaps drill a small pilot hole from the attic in the one spot where you know you want a light... and then use that hole to locate yourself in the room downstairs and take all measurements from that one hole. I would not drill pilot holes for all lights from the top because it will be more difficult to get them perfectly aligned and evenly spaced.

As a final point, make sure you use the correct pots for the ceiling you have. There are plain pots, pots for insulated ceilings, and pots for insulated ceilings with an integral vapour barrier built in. There are pluses and minuses to each of them: plain pots get quite hot and ~cannot~ touch any insulation; while insulated pots take up more space and may be more difficult to install where you want because they interfere with the joists. We typically use the pots with integral vapour barriers because its way easier to insulate and put a vapour barrier up.

Home Depot has a hole cutter in the tool section with a carbide edge that is made for cutting holes for recessed lights in sheetrock & plaster. It's made by Remgrit. I have cut over 50 holes and it's still like new.

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Then get a pie plate, punch a hole the size of the mandrel, 3/8", in the center. Mount it between the drill and cutter, gives you a dust catcher.

Never use a reciprocating saw as was suggested on plaster. Plaster is laid up in two layers and it is very easy to crack the plaster with a up and down motion and cracked plaster is very expensive to repair properly. Need to think about how to cut holes with cost of a repair (new plaster and paint) in mind. Safest way to avoid damage is to use a circular cutter and for plaster it needs to be carbide tipped at a minimum and ideally get one that uses tungsten carbide grit and has a continuous rim. Problem is that the carbide grit hole saws come in a very limited number of sizes (3-1/8, 4-3/8, 5-3/8, 6-3/8, 6-5/8, 6-7/8) and the light cans you find at Home Depot or Lowe's need cutout sizes of 3-3/4, 3-7/8, 4-1/4, 4-1/2, 5-1/2, 6-1/4, 6-1/2 inches.

I have had the most success (cutting thousands of holes for high hats or recessed light cans) using a Hole Pro adjustable hole cutter with the tungsten carbide blades on sheetrock or plaster or Hardie board. The X-230 model I use infinitely adjusts from 2" to 9" so I can make the hole the size I want and can install 3, 4, 5, 6 inch recessed light cans with the single adjustable hole cutter instead of 4 or more carbide grit hole saws (more expensive even if they existed in the sizes I need).

The Hole Pro shield catches all the dust and I can set the cutting depth so I do not cut wires up in the ceiling which might also slow me down or add to the cost of putting in the light cans.

Couple tricks I have learned over the years. First is to drill a small pilot hole and use a coat hanger or similar gauge of wire bent into an L shape and poke it up through the hole and spin it around. Makes it easy to check for the joists and that the hole you make for the can light will not be hitting it. If you hit a joist then move the hole in the direction needed to miss the joist. As the first pilot hole will be inside the circle for the cutout there is no need to patch and if it is outside the new location a 1/4" hole is quickly fixed.

Second trick with plaster ceilings is to cut the first hole only through the plaster and not through the lath which can be woven metal, wood, or "chicken wire". I cut the plaster out with the Hole Pro hole cutter and then pry out the plaster. Because I am prying only the center piece if it cracks or breaks it does not matter. Depending upon the lath I find I will cut it out with a reciprocating saw with a carbide grit blade if the lath is metal, with a fine tooth reciprocating saw blade if it is wood, and with wire cutters if it is the chicken wire. I have a clean hole cut in the plaster so I can use it for my template to quickly cut a hole in the lath material.

There are electricians still using sawzalls or rotozips to cut holes for light cans but it is a slow and messy way to go and with plaster the risk of cracking the plaster makes a sawzall a foolish way to go as well. Difference between the average pro and the average DIY is a combination of experience (learning from mistakes made) and better tools. You can improve your chances for success with better tools even if you have little experience.

Construction work should follow the old medical adage of first do no harm but it often gets lost in the bad advice given.

The Ridgid Bi-Metal hole saw will certainly last for 9 holes but I think you'll find it's expensive. The bi-metal is simply high-speed-steel teeth fused to the circular carbon-steel body. No need to make the whole thing out of high-speed-steel when it's just the teeth that need to stay sharp. Use the Remgrit, it was made specifically for this job. It will make the least mess (dust) too.