I also go straight into 5 gallon buckets when I grow in dirt, no problems. Hes going in a tent so you also need to subtract the bulb and add space between the bulb and the roof of the tent or hes going to start a fire. I love my 600,s but wouldnt even attempt it in a tent that short. Flouros or led would be my suggestionI start plants in 5 gallon pots all the time. I see great growth before the plant's roots fill out the pot. There is no such thing as too big a pot.
Is the Earth too big a pot?
A Root bound solo cup will slow growth, a big pot will not.
Plus, I dislike transplanting, so do the plants.
1.5 meters with a 600 watt light is cutting it close.
My plants need to be about 16" away from my 600 watt light to avoid bleaching.
1 foot for the pot
1.5 feet for the light
That leaves you about 2 feet for the plant.
I would suggest you start flowering when your indica plant is one foot tall.