Depending on your router and the number of devices connected to the hotspot and what those devices are used for, you could run into bandwidth problems (not enough resources to go around, which would slow down everyone connected). In general, what you connect to your side of the router is your business. As long as the bandwidth you are paying for is adequate for your purposes, you can use it however you want. You are likely to find that ten people can't successfully stream different live movies simultaneously on a typical home Internet service, for example, but configuring a hotspot shouldn't affect the charge for your service. Your Internet Service Provider will be happy to upgrade you to a faster service at a higher price if you need it to support your hotspot.
"Safe" is a relative term. Opening your network to anyone within range is not as safe as authorizing each user/device allowed to connect. That isn't practical in a public location, like a cafe, but it is a one-time thing you could easily do for, say, each family member and their devices. When a cafe or airport sets up a hotspot, they dedicate an isolated computer and network to it. They don't run their business on the same computer or the same network.
