12ax7 tubes can be enterchanged with xxx tubes?

5751 should work fine. 12AT7 might work. I wouldn't try 12AU7.

I like the 1950s GE and RCA 3 mica black plate 5751s.

Can anyone provide a good source for the RCA 5751 triple black plates. I ve seen them for sale by upscal audio 50/ea. is this the going rate? do i need to purchase the gold version for xtra 25/ea for my application?

$50 for a more or less gaurunteed tube of that age and scarcity is a very reasonable price. In that application, I really don't think you need to spend the extra money for the quieter tubes. Obviously YMMV etc. try the standard grade and see for yourself.
FWIW The RCA triple mica blackplate is indeed a very good 5751 but not particularily warm.

IMO:
So many really good choices in vintage 12ax7 that I would recommend trying:
Almost any 50's or 60's vintage USA made 12ax7 will sound better than current production ones. Try RCA or GE or Sylvania grey plates also...not quite as refined as the soo sweet black plates of that era, but still very good, and don't command the premium prices either.

Holy grail 12ax7 for me: Telefunken smooth plate, closely followed by the ribbed plate:

Any Mullard or Amperex from that era:

I have 15 12AX7WA Sylvania, USAF stamped mil spec tubes dated 12-64 that are some of the finest tubes I have...absolutely smooth, very quiet, with very fine detail that I paid $5/ea. for....so you can find super tubes for fair prices if you just broaden your search field a bit.
Good Luck....tube rolling is great fun!

Twinbugles would you kindly share where it was that you found $5 12AX7s US made in the 1960s. I would like to expand my search to places offering such bargains. Can you Help me out? If they also have RCA 1950s triple mica blackplate 5751s for a similar price please pass that along as well. Actually prefer Sylvania but any brand of 5751 would be fine Thanks in advance.