All applicants meeting the priority deadline are given full consideration for assistantships – you do not need to apply separately.
Decisions on GRA and GTA positions are separate from admissions decisions. Decisions about assistantship positions for fall admission are typically made in early March. However, new funding becomes available throughout the calendar year and thus additional assistantship positions may be offered in the months prior to the start of fall term. After being accepted into the graduate program, it would be appropriate to contact faculty members whose research matches you interests to inquire whether they have funding available.