Technically, ALPA has no official policy on it. What ALPA does have a policy on, though, is that contracts should be consistent with what is known as "Decision 83." Decision 83 is a ruling from the old National Labor Board, I believe back in the '30s, that required pilots to be paid by productivity, which at the time used a formula based on aircraft speed, and which was later updated to include aircraft weight and other factors. The idea being that a pilot who flew a B707 was far more productive for the airline than a pilot who flew a DC-3, because he generated a hell of a lot more available seat miles. Decision 83 came into existence a long time before jets came on the scene, so when jets started showing up at the airlines, it was a huge windfall for the pilots, because their productivity (and therefore their pay) skyrocketed. That windfall that was the end result of an NLB ruling that Captain Behnke pushed for so many years earlier helped propel airline pilot compensation during the regulation era to incredible levels that we've never seen since, and probably never will again.
Section 40 of the ALPA Administrative Manual, which is where Association policy on collective bargaining is located, contains the requirement for contracts to be consistent with Decision 83. It's not more specific than that, though, so you can interpret it any number of ways. Besides, negotiating committees ignore large portions of Section 40 on a regular basis, and no one ever enforces it. If a pilot group wanted to negotiate pay rates that were the same across fleet types, no one at ALPA would stop them. But you may see an MEC or NC that doesn't want to do it use ALPA policy as an excuse, which is really just their interpretation of a pretty flimsy rule.