Rummys are among the larger tetras when they get more mature. But when younger, their shoal should protect them. The way a shoal works is that as the 6 or more rummys move together, the tiny brain of the angelfish interprets the darker colors of the rummys as being a single, moving, large fish or other thing and it leave "it" alone.
Other things I can think of are that if the angels are young, they will get used to the rummys being in the tank (just like they do for neons, which are even smaller) and not eat them, even though they, the angels, have grown larger. On the other hand, one negative posibility, a minor consideration, is that angels, like many other cichlids, are among those fish whose behavior varies some by individual fish.
Bottom line, I'd think rummys would be fine with angels as long as the minimum shoal size (at least) is maintained.
~~waterdrop~~