I'm installing bathroom light, one of those 3 bulb lights, it's going across the tub and it'll be on a dimmer. I'll use small bulbs because that light doesn't need to be strong.

Is 18 gauge/7amp wire good to connect that light?

I'll probably use low wattage bulbs (10-15watts)

Is the dimmer for the light going to be within reaching range from the tub?

If so, you are going to have to put the circuit on a GFCI breaker for safety reasons. Also, general lighting uses a 15 AMP circuit, which requires a 14/2 AWG cable.

It's going on GFCI, and I was just thinking (since I have a lot of 18 gauge/7amp cable) to use it since it'll be 3 very low wattage bulbs.. any way to check and make sure this cable would be OK?

The cable is not OK. It is not permitted as permanent wiring. The insulation is not approved for permanent wiring. The size of wiring must not be smaller than the size approved for the breaker that protects it.

Suppose a short circuit developed in that wiring. The insulation would burn up and start a fire long before the breaker tripped off. Don't care how much you have, you can't use it.

GFIs don't protect against overload.

The cable is not OK. It is not permitted as permanent wiring. The insulation is not approved for permanent wiring. The size of wiring must not be smaller than the size approved for the breaker that protects it.

Suppose a short circuit developed in that wiring. The insulation would burn up and start a fire long before the breaker tripped off. Don't care how much you have, you can't use it.

GFIs don't protect against overload.

Ok, so what cable do I need (gauge/amps)?

What size is the breaker for that circuit. Fifteen (15) amp requires 14 gauge wire, 20 amp requires 12 gauge.