Is that 3.5 long 1.5 deep 4.5 tall? If so how to you plan to grow? SOS, SCROG,ect.
Not shur about cfm of growbrite fan. But you have 25 cf of grow area. Need to exchange air every 1-3 minutes so fan needs to be 25 cfm and up.
Why do you what to use both? A 250 hps is enough light for that room that's 47-48 watts per square foot. So heat will be less also. Well hoped this helps.