I'm glad to see you are doing a microbiology project Some tips for culturing samples:
1. Use a sterile cotton swab to collect the sample.
2. Practice aseptic technique. Try not to expose the agar to the air while streaking the plate. An easy way to do this is to set the plate on a table, lift the lid with one hand just enough to get the swab inside without touching the outside, and use the other hand to transfer the sample from the swab to the agar.
3. Feed the test subject a treat AFTER collecting the saliva sample.
The types of bacteria in each saliva sample will be a little different. Try looking up the properties of each species' saliva. What is different about each species' behavior? For example, cats are obligate carnivores that require much higher dietary protein than omnivores like dogs and people. The diet, age, and other habits of the test subject will influence the types of bacteria you find.
Could you be more specific about your experimental conditions? Bacteria isolated from living things like body temperature. Is your incubator close to that temp? A little lower is OK, but the bacteria will grow slower. Some may not grow at all. What type of agar will you use? Nutrient agar will support some types of bacteria but not others. Most bacteria isolated from mammals like sheep blood agar. In a research or diagnostic laboratory, we incubate for 24 hr or 2-3 days, depending on what we're culturing and the tests we need to do. In class labs, we would incubate 3 to 4 days.
Also consider sources of contamination for your project. Bacteria from your skin or the air may be transferred to your plate if you aren't careful. You could try a control plate for each to identify potential contaminants for your project. Swab your hand and streak one plate, leave another open to the air for about 10 minutes. Check your plates every day. Mold can grow within a few days and mess up your results.