OKay, so I know that everybody has a point when they just can't put on muscle anymore, but does that mean for a certain area, or for the whole body?

Like, to keep it simple, say you lift weights for 30 minutes everyday, you do all sorts of machines, including abs, which you do 70 lbs on, you continue doing that with all the other weights you're doing until you get to the point where you can no longer build muscle and you can do 85 lbs
OR you go to the gym to ONLY do ab machines, and let the rest of your muscles stay weak- would you gain more muscle than you would if you were working the rest of the muscles? Would you be able to do more than 85 lbs?

If I'm not making my question clear enough, I'll try to explain it again
I'm just curious, because I work a lot on leg muscles, and some ab and arm muscles, but not so much shoulders or back, and i was wondering if I started doing those, would I lose muscle to gain it in those spots

Not if you continued to work them. If you stopped those exercises then yes. Maybe you might like more of a total body approach. If you're bored with what you're doing or need new ideas you might want to try free weights.

First, I doubt most people come anywhere near their maximum.

Second, I dont think it is a matter of "you get X lbs to distribute over your body" My guess is it comes closer to each particular muscle having a limit.

In general though I would say the exact opposite of your example would be true. Most muscles work in concert with each other. Although in some machines you can truly isolate, almost every movement calls some other muscles in to play. You will limit your ability to max out any muscle by leaving the other muscles behind.

Instead of doing isolation exercises, why not go for full body exercises instead. Squats, lunges, deadlifts hit several areas of the body at once and you will be workign them all out.

THere are many of the upper body workouts that will target your arms, shoulders and back. Pullups - assisted pullups will work your arms as a secondary muscle group.

Get diversified. You probably will not lose muscle mass that you have built up so far as long as you continue to work those areas as well. Although, if you do some of the full body, multi-muscle exercises first then try one of your isolation exercises, your muscles may be more fatigued and you may not lift as heavy as you would during isolation. But that doesn't mean you are weaker, it means you are working those muscles harder and over time you will see overall improvement.

I'd second the recommendation on full body/body weight exercises. My suggestion would be to avoid machines at all cost, no real use of isolated your muscles as it is less effective plus they often put your body in an unnatural position.

Thanks!
I do everything except for shoulders and back, really, so I plan on starting those once I'm back from vacation ( no point in starting now if I wont be able to work on them for 2 weeks)
I also heard that rotating machines each day is good, so maybe I'll doing lower body and upper body every other gym visit, that will keep things more interesting anyway
And I'll start eating more tofu (yum!)