Welding & NDT

This is fastener standard for bolts
Grade 8.8 means, Approx. 800 Mpa tensile strength and 640 MPa yield strength (you will arrive at the YS by multiplying both the numbers times 10. That is 8x8x10=640)
Similarly, 10.9 means approx.1000Mpa TS and 900 Mpa yield strength. See the below-given details.

Property Class	Tensile Strength, Mpa.	Hardness	Material	Heat Treatment	Tempering
4.6	400	B67-B87	Low or medium carbon steel with max limits for C 0.55, P0.048, S0.058	No
4.8	420	B71-B87
5.8	520	B82-B95
8.8	830	C23-C34	Medium carbon steel with max   limits for C0.28- 0.55, P0.048, S0.058	Oil Quenched	min 425°C
9.8	900	C27-C36			min 410°C
10.9	1040	C33-C39			min425°C
12.9	1220	C38-C44	Medium carbon steel with max   limits for C0.31- 0.65, P 0.045 MAX, S 0.045MAX	Oil Quenched	min 380°C

YES. Refer Section IX part QW 461.9
YES. both belong to group F4. Refer Section IX  QW-432 (Table)
As per Section IX QW-253, it is acceptable. But ensure that your project specifications won't put any restrictions to do so.