Clowns are some of the very best tankmates with cichlids, and work with all but the most aggressive ones. They are largely ignored. I house them with a large severum, very large oscar, Carpinte, and have had them with some more aggressive ones. They work great, and are very active in the tank with even the most boisterous cichlids. They do grow slow, so will be fine in a 125 for several years, and lage clowns actually are saleable because of the rarity of the big boys, so if they outgrew the 125 at some point, you can trade them with limited trouble. A very good choice. Keep the water clean and they will do well, and are a great addition.
BTW, the guy at cichlidscene (.com) has an incredible collection of clowns, kept with cichlids for the most part.