Do not combine with waste vent, radon vent, gas appliance vent, or really any other type of vent.
A connection to an existing vent could allow back-flow even with a booster fan.
If connecting to a waste vent stack you could expose yourself to these bacteria (like legionnaires disease).
Connecting to a radon vent is also a no go. Radon is very heavy compared to air and the Bernoulli effect and/or a booster fan can only work to lift the radon if the top is the only path out. Otherwise it will always drop to a lower level, defeating the purpose of the radon vent. Usually a radon vent also has a suction alarm to indicate if there are leaks in the pipe which would also cause it to fail.
Connecting to a gas appliance could allow CO and/or gas to back up into your laser/workshop. Turn on the laser and BOOM.
If you are not using a filter I would recommend a unique vent for the laser that goes up through the roof, out of a side wall, or out through an eave. If the length is long enough to warrant a booster fan I would place the fan near the end of the run in a location where it could be serviced (unfinished attic space for example) or add a service hatch. I would use smooth duct for all ducts that are hidden in a wall or floor (flex for the exposed part of the connection). The system should be designed in such a way that it can be cleaned with one of those pipe cleaner looking duct brushes.
I have posted here about how long of a duct run could be before you would need to use a booster, and how to calculate the effective duct run.
Not much is really know about how best to vent these, as the 
is the first large scale deployment of lasers for home use. What I recommend might be overkill (I have been called alarmist by others on this forum), but I would rather be safe than sorry.