1 grain is a good starting point. Make sure that your doctor now adjusts the Armour by free t4 and free T3. While the TSH was OK for adjusting the Levoxyl, it is not OK for adjusting the Armour. If you feel fatigued before you are suppose to test (usually 4 weeks on Armour) make sure to call your doctor as you may need an increase sooner. Also, remember to test first, then take your Armour. Morining testing is recommended.
Yes it is OK to start on the one grain compared to 75mcg's of Levo.
I suggest though - since you now will be on a T3/T4 combo medication - you may want to consider splitting the dosages through out the day.
Taking 30 1/2 grain - in the morning and another 30mg in the noonish hour should not put you in a jittery situation - if you were to take it all at once.
Having direct T3 is different and there are things that change when going on this type of treatment.