SUM() the result of a function
So, I found a workaround...

First I made a proxy function on which the first proxy function will run :

CREATE OR REPLACE
FUNCTION p_count_toto(
    in_id_toto INTEGER
) RETURNS INTEGER AS $func$
BEGIN
        RETURN (SELECT sum(tmp_nb_toto) out_nb_toto FROM count_comm(in_id_alias));
END;
$func$ LANGUAGE plpgsql;


And I modified my first PL/Proxy function which looks like this now :

CREATE OR REPLACE FUNCTION count_toto(
        in_id_toto INTEGER,
        OUT tmp_nb_toto INTEGER
) RETURNS SETOF INTEGER AS $$
        CLUSTER 'main'; RUN ON ALL;
$$ LANGUAGE plproxy;


I think that's not the most elegant way to do it, because at the application level, devs will have to change the name of the procedure they'll call, but it does the work.