Claud Butler Tandem - add a front derailleur?
You may have a few troubles with the curved stoker seat tube, front derailleurs can be remarkably tricky to set up correctly. You can find some more useful experiences here. In particular, you may have to experiment with some low cost parts before committing to something pricier as your choice of chainring sizes will determine where along the curve the derailleur must sit and so what effective angle it will be mounted at.
The maximum allowable chainring size will be determined by the length of your bottom bracket spindle and the clearance of the chainstays. Either crack out some geometry or try it out. It's always tricky to work out chainlines on older parts!

In answer to your second question, your best bet is to have a look here and compare the measurements they have to your frame with a pair of callipers. SJS sell the Thorn tandems so always have a good range, you may have some luck with Tandems East if SJS can't help. Although they don't list the sizes they do offer custom EBB inserts (from the US, at a high cost I expect).