1. Problem: Early game vs MG spam (especially when theyre in buildings)
As USF. Ignore the MGs, harass, and/or ask your allies for indirect fire. You don't have the tools to deal with a lot of mgs in houses early. No point wasting manpower and munitions to clear them out with smoke and grenades. Go for their cutoffs instead. Smoke points and decapure them to harass. Or tech to a captain for the howitzers, and scout with your rear echelons or a weak squad, to get intel on MG positions. Without losing too much combat power.
If your teammates are helpful, and you let them know, they'll send a mortar or two your way. But it will take a while to clear them out. So stay mobile and harass! It's the best you can do, until you get your tools.
As Soviets, don't attack them with your infantry, let the mortars do the work for you. Smoke the points again, and decapture them. Shell the MGs, force them to move. Put your own MGs to cover the decaptured points. Let them come to you. It'll be a slow advance. Try to harass their cutoffs to force some of their forces to react, and then attack. Take it easy.
2. Problem: Hoards of Infantry in one giant Blob.
Two Mgs and a scout with some infantry screening should do. Keep the MGs back, lead the blob into them with the scouts and then hit them with the infantry as they get suppressed. Ideally.
You can try to split the blob by splitting your own forces and go for points/cuttoffs. Should net you some captures if he/she keeps blobbing. Just remember to slam the retreat button if you see the blob coming. A greater chance for your squad to survive that way, until you get some suppression.
If that's not your style, you can face them head on. Use cover, and make sure you have more firepower than they do.
Then there's the guerrilla warfare with snipers. Stay at range, pick off one at a time and move back. Have some infantry in cover as a screen if possible.
A scout will help. Like the utility car, scout cars, vet 1 Line infantry or the like. You'll be able to see the blob before it sees you and plan ahead. Fight, or avoid.
That's all I can think of for now. Hope it helps you!