Whats the rules of adding a template to the possible familiar? I'd figured instead of getting a Winter Wolf at CL 7, i could get a Half-Dragon Winter Wolf at CL 10 (7th CL + 3 LA) with Improved Familair feat.
Is this possible? If it has to be homebrewed, do you think adding the template's LA to the minimum CL prerequisite is fair??
No. If you look at the examples provided of improved familiars, there really isn't a strong correlation between the minimum CL requirement and the CR or ECLs of the creatures. It's much more a function of how useful the familiars abilities are to the master. For example, arguably the most useful feature of the Imp and Quasit familiars is their Commune ability. Once per week they get 6 questions. The time restriction, however, makes it more similar to a Divination spell every day than an actual 5th-level spell, so a minimum CL of 7 to reflect the ability to use a 4th-level spell seems justified.
For a Half-Dragon Winter Wolf, the big draw would be the freakishly high 26 strength (for a familiar, at least) and 100' fly speed. If it's a half-red, then it's not even vulnerable to Fire anymore, in addition to the usual added immunities. To compare, a Gold Wyrmling requires a CL of 14, has a strength of 17, and a 200' fly speed at the same maneuverability.
That said, a Winter Wolf also has a requirement of BAB +7, so maybe increasing both the BAB and CL requirements would be best. I'd suggest CL and BAB of 11.