If I understand correctly version of subversion defines:
Most clients will respect the format they find in the .svn directories. That means they won't change it. If they don't support the format, you'll get an error but they shouldn't corrupt the data. If you use several different clients to update/work with the checkouts, they should create locks (so the second client will say that someone else is currently working on the checkout and stop). But to be safe, you shouldn't try this.
Yes, a 1.4 client can talk to an 1.6 server but the new features won't be available.
Usually, the client will say somewhere which server versions it supports but the clients are different projects and have their own version numbering scheme.