The Romulans alowed Sela, whose mother was a human Starfleet officer, to rise to a high rank in their military. They're not as anti-alien as you think.
Yes, in general, they are. They have always been portrayed as at least semi-xenophobic, and certainly
prejudicial towards non-Romulans. Furthermore, there's a big difference between a slave race that looks nothing like your species, and a half-breed who's not only much more visually similar in appearance to your own kind, but who's Romulan father has military clout. Moreover, Sela's character is quite clear about which half she's most loyal to and identifies with - the Romulan
This is not the case with the Remans, and Shinzon is 100% about them, NOT Romulans or their concept of the Empire.
Shinzon and his Reman group also had the support of the Romulan military, who wanted the current leaders gone in order to make war with the Federation - a war that needed the Remans (who were retconned into the Dominion war as foot soldiers) to succeed.
Which made ZERO sense. Nevermind that the Reman concept was never needed before, but it's quite clear from past shows/depictions that Romulans don't negotiate from positions of weakness, which by making slaves and a clone the head of state, relegates them to subordinates, i.e. it puts them (Romulans) in a position of weakness by allowing slaves to have control of highly destructive technology as well as the government and military.
In other words, the Romulans in Star Trek Nemesis are idiots who behave like atypical Romulans. Maybe they were from that mirror universe of one of the novels that claimed they were suicidal in that reality? That's the only explanation that makes sense given their previous MO, and lack of oogly-boogly slaves-turned soldiers.