All amateur road racing?
Only some amateur road racing?
Does mass start and individual TT make a difference?
Triathlons? All amateur triathlons?
Limited types of triathlons?
1. It depends... some amateur races will require the bike to follow UCI rules. You would need to check with your national and local cycling organizations (it may be different at different levels of racing...)
2. See above
3. The rules are commonly different for TT's and mass starts. You may be allowed to use a TT frame as a road bike, but you can't use aero bars, etc. Again, see #1
4. Triathlons are commonly very much open to anything that has a brake... as long as the bike is functional and safe, you're good. The rules are slightly different if you race ITU though (aero bar extensions cannot go past virtual line between front of brake hoods etc.)
5. See #4
In general, beam bikes are good for comfort, and they can be very aerodynamic, but they do tend to be very heavy, and the beam tend to flex if you try to "punch" up a climb and/or accelerate hard etc.