Is it plagiarism if you combine different algorithms to form one algorithm?
Combining various things to form one new thing is valid research. If your algorithm is a hybrid of 2 others, you need of course to mention these 2 and their authors as well (which you seem to indicate that you are doing). Otherwise you risk your readers thinking that everything in the hybrid algorithm is new, especially since they were developed way back. 
Long story short, if you're acknowledging the work you are basing your approach on, you're being a good student. No plagiarism there.