Hi.
Generally, all European data SIM cards or "data-only" plans work in all GSM devices: smartphones, routers, moderm, Mifis, tablets ..... They are targeted at tablets and modems. So there is absolutely no problem to use it in a modem creating a WIFI hotspot (Mifi router). But you can use it in smartphones too, only without (regular) voice calls and sometimes texts. It is even easier sometimes to book a data-only package through a phone by USSD code than to navigate on personal online accounts on the web in foreign languages.
Whether the specific provider allows VoIP calls is a totally different story. T-Mobile in the US actually pushes Skype, while the same company in Germany on its home network actively blocks it. The telcos (or cellcos) don't like VoIP so much over here
There are 3 attitudes:
- banning VoIP calls by blocking the respective ports (this is rare, but happens)
- banning VoIP calls in their T&Cs only - with no consequences for the user at all, you can still use it
- allowing VoIP - which is the most common and widespread in the countries you visit.
So for Skype or other VoIP, you better ask in the shops of the operator when you buy your local SIM card, as their policies sometimes change. But normally, you can go ahead. For a decent Skype call, you should have a stable 3G or WIFI connection. That's why you should prefer one of the bigger networks on the list to the smaller (and often cheaper) ones.