The unit comes with a two user license to the "Hoblink" java client so you can always try it out at no charge (refered to as the Premier Java RDP Applet.) Within your RDP configuration (resource profile) you specify a Java client that can be used as primary or fallback. So in that sense you can use as many different Java clients as you want but you can only have a single client defined for a given resource.
Usage is driven by resource, which in turn is driven by role so user access is of course totally controllable.
There is no, one single free Java client. There are several out there that work to differing levels on differing platforms. Free clients would need to be evaluated based on platform and functionality requirements.