want to lower ceilings to 9' from the 9.5' they are now by running 2x6s 24" oc at right angles to existing joists. These will tie into rim joists and also be supported from the existing joists . On a 12 x 15 room the total ceiling weight would be only around 500# so I believe this would give much more than enough support for new ceiling. Thank you for your input.

These will tie into rim joists and also be supported from the existing joists. ​ How will they be supported if they are below the existing joists? Lot of work for a 6" drop. Are you going to use a finished ceiling? Drywall?? If not, why not just use a dropped ceiling?

Like I said , the 2x6s at 15' will be right at max span and will use rafter tie or similar at center point to tie to existing joist for added support though they will be supporting only like 2# per sq. foot drywall weight. This is actually easier than tearing off the old lath and plaster ceiling with the resulting mess and still give me the 9' ceiling I want replacing the old 8' dropped ceiling. Will then be using 54"x12' lengthwise on walls to meet the 9' ceiling. I think I have answered my own question but input still appreciated. Fyi , this house is like 125 years old.

What's above this level?
I would hire a demo crew to make a mess tearing down the plaster and lath.
Insulate, insulate, insulate that 125 year old ceiling.