Plumlee is a rim protector IMO or at least way better than Monroe is, but that isn't saying much because Monroe is terrible in that aspect. Also it depends on which Miles we get, because for the first 2 months of the season he was terrific at defending the rim, (2 BPG and a good shooting percentage allowed at the rim) but he tailed off as we got deeper into the season. He is really good at going straight up and not creating contact and thus making it very hard for guys to get their shot off over him. Hopefully he is conditioned and ready for big minutes all year this year.
As for Ibaka, "stretch 4" may have been a misleading term as that is generally linked to 3 point shooters, though he does have the ability to shoot 3s his range is typically around 17 feet. As for him fitting with Monroe it really comes down to the way a team plays offense. The Suns like to fill the court with shooter to stretch out the defense for the guards to penetrate and ultimately score. In this situation you just can't ever have Monroe and Len/Plumlee out there together because our guards have no chance of consistently getting to the rim. However defensively you can't afford to have him play big minutes along side Keef and Tolliver because there would be little to no rim protection.