Whenever running Rolemaster in the Forgotten Realms setting, I just use the FR maps, deities, political and religious structures, etc. Then I use the straight from the book RM races, creatures, professions, spells, magic items, etc.
This can cause a little adjustment for the players who are used to the drow and all their overpowered silliness. The RM Companion I has stats for dark elves or you can use the stats for Dyari from Shadow World. But usually, it works just fine. If I want to use a D&D specific monster for a little flavor, I just wing it.