According to Hardware Corner here, the 6400 lp is one of the compatible GPU with the stock PSU.

I Found that but its just because other people said they had issues with the higher power draw of the rx 6400

Due to the limited PSU, the 5050 spec sheet only has a couple of options for GPU's:

I'd only use the R7450 in the MT since it's 65w.

More ideas: 

• R5 430 = 50w.
• GT 1030 = 30w
• Quadro P400 = 50w
• Quadro P1000 = 47w

Though it's within Dell specs, It's not surprising to me that some say 50w RX -400 is too much for 180w PSU.

so if i replace my 5050 sff psu with this one then install a rx 6400 lp it will all work?

is there any psu upgrades i can use instead of a diffrent gpu?

also what about the extra connector will that stop the psu from working if its not pluged in?

Dell 360w TFX psu with 6+4 pin is a good upgrade for 5050 sff.  Recently two users had validated Dell 500w TFX psu with 6+4+4 pin in 7050/5060 too.

https://www.ebay.com/itm/Genuine-Dell-Optiplex-7060-MT-360W-Power-Supply-19YR3/175679587928?

MODEL H500EPM-00

UP/N: D500E005P

Dell P/N: Y7R0X

also will all of those gpus fit in the full lenght slot?

This topic had in length discussion before.  5050 sff uses same chassis as 7050sff

https://www.dell.com/community/Optiplex-Desktops/Dell-Optiplex-7050-SFF-Best-video-card-alternative-for-gaming/td-p/8107869

7050 SFF has same chassis as 7070 SFF.  Watch this video and its part list which tells you best currently available gpu list that is physically compatible with the SFF case.

https://m.youtube.com/watch?v=LVZJ4zc1oRU
List video cards will work with this system.

1.) GIGABYTE GeForce GTX 1650 OC 4GB, GV-N1656OC-4GL Video Card

2.) Gigabyte GeForce GTX 1650 OC GDDR5 Low Profile

3.) Gigabyte Geforce GTX 1050 Ti OC Low Profile 4GB GDDR5 128 Bit PCI-E Graphic Card (GV-N105TOC-4GL)

4.) MSI Gaming GeForce GTX 1650 4GB GDRR5 Low Profile Graphics Card (GTX 1650 4GT LP OC)

5.) MSI GTX 1050 TI 4GT Low Profile Graphic Card