Unite is not the correct term, he planned to annex Bulgaria and assimilate the Bulgarians - the Serbian propaganda that all non-Mongoloid Bulgarians (ie all of them) are actually Serbs dates from this period.
Really? I'm not doubting you, but can you clear that up? According to what I've read on the subject he managed to reach an agreement with a group of Bulgarian emigres in Bucharest, so they must have found his plan, whatever it exactly was, acceptable. Of course, he could have just lied to everybody, but if he told no one and left no records, how would we know he planned to assimilate the Bulgarians in the first place?
And I also don't remember any Serbian propaganda from the period saying Bulgarians are Serbs...except Crazy Milosh Milojevic, considered crazy for some very good reasons, and not just in Bulgaria either.
And finally, such a codification would destroy the connection between Serbian and Croatian, which would obviously hurt the creation of an united Serbo-Croatian state, an even stronger desire of the Serbian nationalists.
That's the smallest of all problems involved, the desire for an unified Serbo-Croatian state was only codified in the 1840s, during the reign of the rival Karadjordjevic dynasty.