First question: since you're using RH, do I assume correctly that you intend to upgrade from rpm?
Second question: Not a question really, just an observation. The libs ought to be backwards compatible. Anything which requires 1.2.10 should still work under 2.4.0, just can't use the added capibilities.
Third question: have you considered using rpm -U to upgrade an existing installation, rather than rpm -i to make a parallel installation and possibly create a host of conflicts with two installations in place?
For more info, study the rpm man pages, or better yet, RH's own Max-rpm, a huge book with all you'd ever want to know about rpm.