Yes give him time to acclimate - about a week undisturbed is what folks usually recommend. If he is that young a full grown mouse MIGHT be too big (mice fade from memory sorry). In general if the mouse/rat is about the same girth as the BP you're good to go.

I am not sure I follow on the 1.2 being too much. Is that a gender breakdown or do you have a fifth of a mouse laying about? Mice and rats are both fairly easy to breed - however mice are much much stankier than the rats.