Bicycle components question - shimano sti shifters (ultegra and tiagra)?

I have a 2008 p2sl with ultegra components on it. 2x10 speeds. It is currently a tri setup but I am looking to switch it to a road setup. I am going to purchase the tiagra sti 4600 shifters that are 2x10. I was wondering if it is ok to mix and match parts like that? Also I have the option of getting the rear... show more I have a 2008 p2sl with ultegra components on it. 2x10 speeds. It is currently a tri setup but I am looking to switch it to a road setup. I am going to purchase the tiagra sti 4600 shifters that are 2x10. I was wondering if it is ok to mix and match parts like that? Also I have the option of getting the rear Derailleur that matches the shifters. Should I get them to replace the ultegra derailler or just get the to have a spare because the derailler is only an extra $10?

Thanks in advance.
Update: The only reason I want to know about the derailler is because it would be newer and I wanted to know if it would be better since im changing the shifters. currently i have the end bar shifters that are separate from the brakes but I want to make it a road bike setup with the sti shifters and I found a cheap used... show more The only reason I want to know about the derailler is because it would be newer and I wanted to know if it would be better since im changing the shifters. currently i have the end bar shifters that are separate from the brakes but I want to make it a road bike setup with the sti shifters and I found a cheap used pair of tiagra shifters.
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