What is a good choice for enlarging lenses if I plan to make large prints, 11x14 and 16x20 from 6x6 negative? What is a good enlaring lens for 35mm?
Does the light source make a difference?
Can I use LTM 50mm and 135mm lenses?
The light source doesn't make a significant difference. I'm assuming by "incandescent" head you mean a condenser head with an incandescent bulb. That will give slightly higher contrast than the mostly diffused light from the 45a head, so unusual lenses with very low contrast might be an issue with respect to the comparison between the heads, but that is for unusual lenses.
A good quality, 5 or 6 element lens in the 80mm focal length range would be ideal for 6x6. Rodagon, Schneider, Nikkor, Minolta, Fujinon, even some of the re-branded Beseler and Vivitar high end lenses would be excellent candidates. Just be sure to be careful about condition, and to avoid the lower end ranges within each line. In some cases, the naming systems can be confusing.
You can also step up to the much more expensive "APO" lenses. They can be superb, but you may not see a big difference unless you go bigger than 16x20.
Some will argue that it can be advantageous to use a slightly longer lens (e.g. 90mm). If you do, you need to have a long column on your enlarger (the shorter the focal length, the greater the magnification).
As for LTM lenses, while they might function, they probably wouldn't cover the 6x6 negative, and they aren't optimized for macro applications (which is what an enlarger does).
In my case, I use a later model 80mm Rodagon for 6x6. In addition to good optical performance, I like the convenience features like illuminated aperture indicator, flexible aperture control and releasable click stops.
