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Yes. This article will help: https://www.gardeningknowhow.com/edible/fruits/raspberry/pruning-raspberry-bushes.htm

It really doesn't matter as long as they have adequate growing conditions. This article will help: https://www.gardeningknowhow.com/edible/fruits/raspberry/care-of-raspberry-plants.htm

If you are seeing ants, it probably means you have an aphid problem. The ants are not a problem, but the aphids are. Once the aphids are treated, the ants will leave too. Here is some information on treating aphids:
https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/get-rid-of-ants.htm

It sounds like you had a small rodent also enjoy your red raspberries and then deposited the seeds all about your garden when Mother Nature called.

You can remove the seedlings and then try to take steps to protect the fruit from other animals that may seed your garden in the coming year. Bird netting, partially buried fencing or hanging old CDs around the plant (which will move and flash in the wind and sun) will help prevent pests from nibbling and later planting new raspberries.

You can plant them within the same area as long as they are all domesticated varieties and certified virus free. Do not plant wild varieties of any kind of berry near domesticated ones as the wild varieties may carry diseases. Wild blackberries can carry fungus and viruses that can make your raspberries sterile (more information here: https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberries-not-fruiting.htm).

Raspberry bushes self-pollinate so only one plant is necessary to get fruit; however, you will get bigger and more productful fruit by adding another one.