I would like to know if Simpex triggers work with Yongnuo flashes.
This question is entirely dependant on what Yongnuo flashes you intend to use. And what Simpex flashes you intend to use.
In a basic sense both the Yongnuo and Simtex triggers should would perfectly fine together providing that the Simpex triggers have either a standard Female Hotshoe.
Or a PC sync socket. (This is the plug on the right of the image).
If you are looking for more advance functions such as wireless TTL or Flash Power Adjustment then the answer becomes more complicated.
You will firstly need a TTL enabled Yongnuo flash and then have TTL compatible Simpex triggers. Please note the triggers and flashes will have to be for the same system. E.g. if you have Canon fit Yongnuo than you need Canon compatible Simpex triggers.
In terms of more advanced functions you may be better off using Yongnuo own proprietary trigger system such as the: Yongnuo YN-560-Tx trigger which will work in conjunction with the built in radios transmitters that Yongnuo is now including with there newer flashes.
For things like HSS you will need to make sure your triggers are compatible of transmitting within 1/8000s of a second.