Replacing a groupset in step purchases
If you want to upgrade gradually you need to follow the compatibility of the groupsets.

If your Tiagra groupset is the 4600 series of 2012, the 10 speed components should be compatible with other Shimano 10 speed road components, so you can use Dura Ace 7800 and 7900, Ultegra 6600 and 6700, and "105" 5600 and 5700.

The amount of cable pull per gear is different between Shimano 10sp road and Shimano 10sp mtb. SRAM 10sp doesn't use the same cable pull as Shimano 10sp road or mtb.