I am running ANCOVA on SPSS using post-operation score (continuous) as Dependent variable, pre-operation score (continuous) as Covariate, and 2 groups (categorial 1&0) as Fixed factors. This is the usual scenario. But can I add another 2 variables one is categorial (gender) and one continuous (age) to the Fixed Factors all at same time? My idea is to check which one of these factors (groups, gender, age) has significant effect and higher partial η2.
Given your comment, yes, this is fine to do, provided the other assumptions are met. You can have any number of independent variables in a linear model and they can be continuous or categorical or any combination.
Of course, you have to have sufficient N to avoid overfitting; you have to check for collinearity, etc, but there is no inherent limit.