They will need to be the same software to link up as the tables will have a different structure for each forum software.
Lets however clear a few myths on 1and1 - although it should be said now, Will and I will disagree on this all night.
for forum size: you are given a 100mb database, enough for anyones books, except if you use it to store pictures - which actally is not recommended by anyone.
160 members, 3,500 threads, 18,700 posts, heavily modded and with chat listings = still below 20mb
email: they have anti spam measures that prevent you sending an email to more than 100 recipients. You just send it twice with 99 recipients each time. No hassle, easy to do in Vb and good practice anyway (we have the same on our email servers at work as it helps admins detect mass mailing worms).
The number of databases is limited by the type of package. Thier 3 main packages are 2, 8 or 15 databases.
They impose the following restrictions, which do work well on a shared host environment. Please note that the MySQL database may not under any circumstances be used for log evaluation operations (e.g., ivw), ad clicks, chat systems, banner rotations, or similar applications putting extreme loads on the database.
ASO are good, but they are non UK, when the transatlantic lines bottleneck thay can be a little slow. I have considered them very seroiusly on more than one occasion, especially when I was toying with the idea of VS, but now 1and1 offer that too.