But the Borg need to assimilate people to increase their numbers, so their population is related to the people available to assimilate. If the Borg do some trial runs and encounter heavy resistance, they make move on to easier targets- the faster the Borg are taken out, the less they can learn, esp. when cut off from the collective. So they may not bother after gaining nothing and having heavy losses.
All the Dominion needs to increase in theory is the genetic Jemmie material and water. Far easier to procure. And they could just have a pilot and a cargobay with something explosive- not that hard. And that could take out 8472 as well. The Dominion still probably has thousands of ships- and the huge ones from the Valiant seem to be pouring out too.
In Star Trek there is not a shortage of people to assimilate. The Delta Quadrant seemed crowded with races. So while the Borg might be restricted to the population of the galaxy, that's still a lot of people. The Borg would have knowledge of cloning, you would think they would also make use of it.
Borg cubes are huge yet 8472 popped them like balloons. Pound for pound 8472 ships would be up there with the most powerful ships we've seen in Trek.
Ramming isn't a bad tactic, wasteful but not bad. We saw a bioship that was most likely destroyed when a cube rammed it. However, it like any other tactic can be countered.
Since the Vorta could be engineered to be telekinetic, who knows what else the Founders can do, how they can modify their peoples. And all a Founder would have to do is get in and damage things. They don't need to be telepathic. They just need to slide through spaces as a tiny rock, break the shields or something, and let the Jem'Hadar do their job.
Given how useful telepathy is as a weapon and tool you would expect that if the Founder's could they would have given the ability to the Vorta. It would make them even more formidable negotiators.
Tiny rock in space meets with ship's deflector and is moved out of the way.
I'm not sure 8472 would bother the Dominion... they'd probably join forces to get rid of the rest of us...
Unless the Dominion attacked 8472 first I doubt they would bother with the Dominion either. Their war with the Borg was provoked by the Borg and they were willing to talk with Janeway. It would seem they don't have an interest in our space and that if we leave them in peace they'll do the same. Unlike the Dominion, which is run by jerks.