You may be comfortable with "contaminating the timeline further" with these ships. After all, it's just a game...
Might as well apply the same logic to the next Trek movie, but bring in Galaxy and Sovereign Classes to fly alongside the "new Kirk" Enterprise. It's just a movie, isn't it? I'm generally not "too much" of a canon whore, I regard Trek in much the same way as I regard most tv-shows, movies and games... Entertainment. But when someone starts to mess around with established canon, in ways that are unimaginably inappropriate, that's when I begin to froth at the mouth.
Oh, and by your logic, we might as well bring in Picard in the next movie, to be a tutor for "new Kirk" on how to Captain the Enterprise. To hell with canon, to hell with established historical "facts". There's a limit to how far you can push canon, in the name of "entertainment".
Long story short, if STO were set in the 29th Century, with all new ship designs BY CRYPTIC, plus the 2 established designs (Wells and Aeon Classes), i'd be perfectly fine with that. As is, the game is set in the 25th Century, where there are NO time ships even invented yet. Bringing a ship from the past, into the present. That's one thing. Bringing a ship from the future into the past, that's a whole different can of worms. People cite the "Mobile Emitter" as evidence, that the Temporal Agents allow future tech to remain in the past. But it's a whole lot different, when they start allowing entire ships. Imagine the HUGE technology boost we would've had on our planet, if someone had presented Einstein with a super computer back in the 1940s. And that's just 70 years ago. Now imagine what a difference a technology boost from FOUR HUNDRED years into the future, would have for effect on us.