advise

toilet bowl clogs ---water comming up in bath tubs --- will NOT plunge ---this suggests a clog in the main line to me.... i ncan't get the "clean-out" cap off to snake it out.... ANY suggestions?????

drain

Even if you could get the cleanout plug off, what would you snake it with? Removing the cleanout plug is usually the easiest part of the job, although without seeing yours we cannot tell you the best way to do it.

Why not use a 1/4" snake and snake the drain through the toilet bowl?

The smallest pipe size for a toilet is 3". A 1/4" snake is too small and flimsy for a drain this size.

clog

You can't snake through the toilet. If sewer goes out high you will have to deal with sewage draining back on you.
If you can get on the roof, run water down that stack and watch to see if it backs up or gurgles in the toilet.
Not seeing the conditions, but you could pull the toilet to snake out or use the stack to snake out.
You didn't mention the kitchen or laundry, so it may be in the horizontal below the bathroom and should be quite easy to open up the drain.