It is necessary to include PCI-E GEN3 everywhere. In the rig with the i3 8400, this changes the speed from 700 to 1000-1100. On the rig with the g4400 processor nothing has changed and the card 1050 gives still 150 h / s. The CPU load is 100% on all rigs and more than 1000 h / s with 1 rig is not obtained. Pentium g4400 it does not work out more than 700-800.
You can put at least as many cards as you like, but you can not get 1000-1200 hashes from 1 rig.
I guess most mainstream motherboards only have gen3 on the first slot.
1 x PCI Express 3.0* / 2.0 x16 slot (blue at x16 mode)
1 x PCI Express 2.0 x16 slot (black at x4 mode, compatible with PCIe x1
and x4 devies)
2 x PCI Express 2.0 x1 slots
3 x PCI slots
* PCIe 3.0 speed is supported by Intel® 3rd generation CoreTM
That would explain the first card being a higher has rate. Jumping from an i5 to an i7 yields a 400h/s increase on the total hash rate.